1. Field of the Invention
The invention relates to semiconductor elements, particularly to avalanche diodes and reverse-biased thyristors, provided with at least one pn junction and with zone guard rings intended to improve the breakdown behavior of the pn junction.
2. Description of the Prior Art
Concerning semiconductors intended for operation with voltages over 1000 V, to improve the breakdown behavior of the pn junctions it is known that at the point where a junction reaches the peripheral surfaces of the element a well-defined surface contour is produced. This is achieved by corresponding surface chemfering angles (Cf. e.g. IEEE TRANSACTIONS ON ELECTRON DEVICES, Vol. E D 20, 1973, pp. 347-352). The production of these structures requires a great deal of technological effort and, for example in the case of negative angles, leads to considerable surface losses.
In order to avoid such surface chamfering angles it is known, e.g. from the book by Blicher "Thyristor Physics", Springer, New York 1976, pp. 231-241, that the pn junction should no longer be directed to the periphery of the element but rather a planar structure should be used with additional zone guard rings. These guard rings are located in a predictable interval corresponding to the planar junction. The pn junction and the guard rings are bounded by the same surface of the element. Through the guard rings the field distribution of the pn junction polarized in the blocking direction is divided into numerous areas whereby the maximum field intensity allowable at the surface is not exceeded in any area. On the one hand, however, such semiconductor elements have the disadvantage that through the planar structure of pn junctions there arises curvatures which on their part limit the inverse voltage breakdown of the elements. On the other hand, for inverse voltages greater than 1.5 kV, numerous guard rings are usually required so that a correspondingly large part becomes lost as an active surface for the essential function of the element.